Infrared heating unit

ABSTRACT

An infrared heating unit with a furnace includes a housing that accommodates a process space, and a heating facility, whereby the process space is bordered, at least in part, by a furnace lining made of quartz glass. In order to provide, on this basis, an infrared heating unit that enables energy-efficient and uniform (homogeneous) heating of the heating goods by infrared radiation to temperatures of even above 600° C., the heating facility is formed by at least one heating substrate that includes a contact surface in contact with a printed conductor made of a resistor material that is electrically conductive and generates heat when current flows through it, whereby the heating substrate includes doped quartz glass, into which an additional component that absorbs in the infrared spectral range is embedded and forms at least a part of the furnace lining.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase filing of international patent application number PCT/EP2017/054846 filed Mar. 2, 2017 that claims the priority of European patent application number 16163897.8 filed Apr. 5, 2016. The disclosures of these applications are hereby incorporated by reference in their entirety.

FIELD

The invention relates to an infrared heating unit with a furnace that includes a housing that accommodates a process space, and a heating facility, whereby the process space is bordered, at least in part, by a furnace lining made of quartz glass.

BACKGROUND

In order to ensure that the process temperature is high while the energy loss is low, the process space is surrounded by an insulating furnace lining, which, e.g. in the case of traditional furnaces, consists of insulating bricks. An electrically heated muffle furnace having a housing that is provided with a furnace lining made of fireclay is known, for example, from DE 1 973 753 U. Infrared radiators with quartz-surrounded heating coils that are arranged at the ceiling wall of the process space are used as the heating facility. However, furnace linings of this type have a high heat capacity, which leads to long heat-up and cool-down times and low energy efficiencies. The use of furnace linings made of fireclay also limits the cleanliness conditions inside the process space.

Such disadvantages are overcome in a furnace known from DE 10 2012 003 030 A1, which includes a furnace lining made up of quartz glass tubes arranged such that they are axially parallel. In this context, the quartz glass tubes are connected to each other on their side facing the process space by means of a connecting mass made of opaque quartz glass, which simultaneously serves as a diffuse reflector such that the infrared radiation is reflected on the bordering walls of the process space. By this means, a high degree of efficiency of up to 90% is attained.

A continuous furnace for heat treatment of glass panes is known from U.S. Pat. No. 4,133,667 A. Infrared emitters are arranged in the process space above and below a conveying facility. Positioned on rollers made of quartz glass, the glass panes are transported through the process space by means of the transport facility. A similar transport facility comprising quartz glass rollers is also described with respect to the continuous furnace known from JP 4715019 B2.

The distance between the heating goods and the infrared radiator plays an important role for the homogeneity of the irradiation. With regard to an axis-parallel arrangement of multiple elongated infrared lamps, an empirical rule of thumb says that the minimum distance required for homogeneous irradiation is equal to approximately 1.5-fold the centre distance of the infrared lamps. Accordingly, a low distance between the individual infrared lamps and a large distance between the lamp arrangement and the heating goods are favorable for homogeneous radiation. The former alternative (close lamp-lamp distance) is subject to physical and technological limits and is associated with higher fabrication costs. The latter alternative (large infrared lamp-heating goods distance) leads to a lower degree of efficiency of the irradiation power used in this process and comparably low radiation power per unit area of heating surface.

The spatial infrared emitter known from WO1999/025154 A1 uses a spatial, planar, tube-shaped or polyhedral heating substrate made of quartz glass that is in direct and continuous contact with an electrical resistor element. The resistor element has, for example, a meandering shape and is applied by means of film, screen printing or thin layer printing to the surface of the heating substrate, and is then burned in.

In this embodiment, the heating element does not heat a surrounding cladding tube, but it directly heats, through direct and extended spatial contact in the form of the printed conductor, the quartz glass heating substrate such that the heat transmission between heating element and heating substrate takes place mainly by means of heat conduction and convection, which can have a positive effect on the homogeneity and the degree of efficiency of heat transmission.

SUMMARY

According to an exemplary embodiment of the invention, an infrared heating unit with a furnace is provided. The infrared heating unit includes a housing that accommodates a process space, and a heating facility. The process space is bordered, at least in part, by a furnace lining made of quartz glass. The heating facility is formed by at least one heating substrate defining a contact surface that is in contact with a printed conductor that is made of a resistor material that is electrically conductive and generates heat when current flows through it. The heating substrate includes doped quartz glass into which an additional component that absorbs in the infrared spectral range is embedded, the heating substrate forming at least a part of the furnace lining.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed description when read in connection with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures:

FIG. 1 is a longitudinal section view of a furnace for heat treatment according to an exemplary embodiment of the invention;

FIG. 2 is a cross-sectional view of the furnace of FIG. 1, including the process space;

FIG. 3 is a detailed view of the bracketing of a support element on a transport belt of the furnace of FIG. 1; and

FIG. 4 is a detailed view of a heating substrate for installation in the ceiling area of the process space in the furnace of FIG. 1.

DETAILED DESCRIPTION

Exemplary aspects of the invention relate to industrial electrical heating furnaces used for the heating of heating goods to temperatures above 600° C. Such heating furnaces often use infrared radiators that emit short-wave, medium-wave and/or long-wave infrared radiation as the heating facility.

The heat transport from an electrical heating element to the heating goods takes place, for example, on the basis of heat conduction, convection and/or heat radiation. One basic issue in this context is how to emit the available radiation power towards the heating goods as effectively as possible (at high power efficiency) and, simultaneously, at high homogeneity. Infrared emitters show point- or line-shaped emission characteristics for the infrared radiation or, as spatial infrared emitters, they show two- or three-dimensional emission characteristics that may be adapted to the geometry of the surface of the heating goods to be heated and enable homogeneous irradiation of two- or three-dimensional surfaces.

Infrared emitters are usually equipped with an electrical heating element made of a resistor material that generates heat when current flows through it. It is common to use tube-shaped infrared lamps, in which a coil-shaped resistor wire is surrounded by a cladding tube made of quartz glass, while keeping a distance from and having essentially no contact to the tube. In other embodiments of infrared emitters, the electrical heating element, such as, for example, a wire, a track or a layer made of the resistor material, serves for heating another, non-electrical, passive heating element, which shall be referred to as “heating substrate” hereinafter.

Quartz glass possesses good corrosion, temperature, and temperature cycling resistance and is available at high purity. Therefore, as a matter of rule, quartz glass, used as a heating substrate material, meets strict requirements in terms of purity, temperature stability and inertness even in high temperature heating processes.

However, quartz glass shows comparably low thermal conductivity and is even commonly used as a heat insulator. If the material is used as a passive heating substrate to be heated by means of a resistor element that is applied onto a contact surface, and if the wall thickness is low, there is a risk that an inhomogeneous temperature distribution on the side of the contact surface may be maintained on the opposite heating substrate side. In an extreme case, the active electrical heating element may be imaged on the opposite heating substrate side. This can be counteracted by a high occupation density of the heating element material, though this is expensive. In the case of thick heating substrate walls, the power efficiency suffers and rapid temperature changes are rendered impossible, as these require rapid heat-up and cool-down of the heating substrate.

An object of the invention is to provide an infrared heating unit that enables energy-efficient and uniform (homogeneous) heating of the heating goods by infrared radiation to temperatures of even above 600° C.

Such an object may be met according to the invention based on an infrared heating unit of the type specified above, in that the heating facility is formed by at least one heating substrate that includes a contact surface that is in contact with a resistor material that is electrically conductive and generates heat when current flows through it. The heating substrate includes doped quartz glass, into which an additional component that absorbs in the infrared spectral range is embedded. The heating substrate forms at least a part of the furnace lining.

In the infrared heating unit according to exemplary embodiments of the invention, the furnace lining includes, at least in part, a material that can be induced to emit infrared radiation upon thermal excitation by means of resistor material printed conductor. Accordingly, this is a thermally active or thermally activatable furnace lining. The material is doped quartz glass into which is embedded an additional component that absorbs in the infrared spectral range, in particular upon thermal excitation. The material may also be referred to, for short, as “IR black glass”, whereby this term does not refer to the visual colouration of the material, but rather to the fact that it absorbs radiation in the infrared wavelength range at room temperature or elevated temperature.

Accordingly, the IR black glass forms at least a part of the furnace lining and, simultaneously, forms a part of the heating facility—and does so in connection with the electrical printed conductor made of resistor material for thermal excitation of the IR black glass. In those regions of the furnace lining that are thermally excited by electrical printed conductors, the IR radiation-emitting IR black glass is a heating substrate and part of the heating facility; whereas the IR black glass is simply part of the furnace lining in other regions of the furnace lining by IR black glass that are not thermally excited. This also applies in those regions of the furnace lining with IR black glass that are occupied by electrically printed conductors and therefore could be functionally excited by thermal means without the thermal excitation actually taking place.

In certain exemplary embodiments of the invention, the furnace lining consists fully of the IR black glass. In other embodiments, only part of the furnace lining consists of the IR black glass. Regardless of the respective embodiment, a single region or multiple regions of the furnace lining are provided as the heating substrate. If the term “heating substrate” is used in the singular in the explanations provided below, the term shall also include multiple heating substrates as well.

Upon thermal excitation by the printed conductor, the heating substrate is therefore the actual IR radiation-emitting element. The heating substrate material contains quartz glass (amorphous SiO₂) that accounts for the largest fraction of the heating substrate material in terms of weight and volume and has a crucial influence on its mechanical and chemical properties; for example, on the temperature resistance, strength, and corrosion properties.

Aside from SiO₂, the quartz glass can contain up to a maximum of 10% by weight of other oxidic, nitridic or carbidic components. Quartz glass possesses good corrosion, temperature, and temperature cycling resistance and is available at high purity. It is therefore a conceivable heating substrate material even in high-temperature heating processes with temperatures of up to 1100° C. The cristobalite content being low (i.e., 1% or less) ensures that the devitrification tendency is low and, therefore, that the risk of crack formation during use is low. As a result, even the strict requirements concerning the absence of particles, purity, and inertness evident (e.g., in semiconductor fabrication processes) are met.

Since the heating substrate may include essentially quartz glass (i.e., an amorphous material) it is easy to shape into a suitable geometric shape for the application on hand, for example, in the form of planar, curved or wavy panels, cylinder or tube shape with a round, oval or polygonal cross-section or bowl, dome or crucible shape.

The additional component embedded in the quartz glass forms a separate amorphous or crystalline phase that is dispersed uniformly or specifically non-uniformly in the quartz glass matrix. The additional component is decisive for the optical and thermal properties of the heating substrate; to be more specific, it effects an absorption in the infrared spectral range, which is the wavelength range between 780 nm and 1 mm. The additional component shows an absorption that is higher than that of the matrix component for at least part of the radiation in this spectral range, in particular upon thermal excitation.

The phase regions of the additional component act as optical defects in the quartz glass matrix, and themselves have a heat-absorbing effect. According to Kirchhoff's law of thermal radiation, the absorptivity α_(λ) and the emissivity ε_(λ) of a real body in thermal equilibrium are equal.

α_(λ)=ε_(λ)  (1)

Accordingly, the additional component leads to the emission of infrared radiation by the heating substrate material. The emissivity ε_(λ) can be calculated as follows if the spectral hemispherical reflectance R_(gh) and transmittance T_(gh) are known:

ε_(λ)=1−R _(gh) −T _(gh)   (2)

In this context, the “emissivity” shall be understood to be the “spectral normal degree of emission”. The same is determined by means of a measuring principle that is known by the name of “Black-Body Boundary Conditions” (BBC) and is published in “DETERMINING THE TRANSMITTANCE AND EMITTANCE OF TRANSPARENT AND SEMITRANSPARENT MATERIALS AT ELEVATED TEMPERATURES”; J. Manara, M. Keller, D. Kraus, M. Arduini-Schuster; 5th European Thermal-Sciences Conference, The Netherlands (2008).

The quartz glass doped with the additional component has a higher absorption for heat radiation than in the absence of the additional component. This results in an improved thermal conduction from the printed conductor into the heating substrate, more rapid distribution of the heat, and a higher rate of emission towards the heating goods. By this means, it is feasible to provide higher radiation power per unit area and to generate a homogeneous emission and a uniform temperature field even for thin heating substrate walls and/or at a comparably low printed conductor occupation density.

The additional component in the heating substrate material is preferably present, at least in part, as elemental silicon and is embedded in an amount that effects, in the heating substrate material for wavelengths between 2 and 8 μm, an emissivity ε of at least 0.6 at a temperature of 600° C. and an emissivity ε of at least 0.75 at a temperature of 1,000° C.

Accordingly, the heating substrate material has high absorption and emission power for heat radiation between 2 μm and 8 μm (i.e., in a wavelength range of infrared radiation). This reduces the reflection at the heating substrate surfaces such that, on the assumption of the transmission being negligibly small, the resulting degree of reflection for wavelengths between 2 and 8 μm and at temperatures above 1,000° is maximally 0.25 and at temperatures above 600° C. is maximally 0.4.

Due to the uniform emission and the high emissivity, the distance between the heating goods and the heating substrate can be kept small, which increases the irradiation intensity and the efficiency accordingly. The distance is preferably less than 5 mm for some applications. For example, in so-called “reel-to-reel” manufacturing processes, in which sheet-shaped or film-shaped heating goods are passed by the heating substrate at high velocity. The distance being low allows for high power densities of more than 100 and even more than 200 kW/m² on the heating goods. The furnace lining with high emissivity contributes to a high degree of efficiency of the heating process. Losses are also being minimized in that diffracted fractions are effectively reabsorbed by the furnace lining and can be emitted again immediately. Non-reproducible heating by reflected thermal radiation is thus reduced, which contributes to a uniform or specific non-uniform temperature distribution.

In an exemplary embodiment of the infrared emitter, the additional component contains a semiconductor material in elemental form, preferably elemental silicon. The fine-particle areas of the semiconductor phase act as optical defects in the matrix and can cause the heating substrate material to look black or grey-blackish by eye, depending on the thickness of the layer. On the other hand, the defects also have impacts on the overall heat absorption of the heating substrate material. This is mainly due to the properties of the finely-distributed silicon phase that is present in elemental form, to the effect that, on the one hand, the energy between valence band and conduction band (bandgap energy) decreases with the temperature and, on the other hand, electrons are elevated from the valence band to the conduction band if the activation energy is sufficiently high, which is associated with a clear increase in the absorption coefficient. The thermally activated population of the conduction band leads to the semiconductor material being transparent to a certain degree at room temperature for certain wavelengths (such as from 1,000 nm) and becoming opaque at high temperatures. Accordingly, absorption and emissivity can increase abruptly with increasing temperature of the heating substrate material. This effect depends, inter alia, on the structure (amorphous/crystalline) and doping of the semiconductor. For example, pure silicon shows a notable increase in emission from approximately 600° C., reaching saturation from approximately 1,000° C.

The semiconductor material, and specifically the exemplary used elemental silicon, therefore have the effect to blacken the vitreous matrix material and to do so at room temperature, but also at elevated temperatures above, for example, 600° C. As a result, desirable emission characteristics in terms of a high broadband emission at high temperatures are attained. In this context, the semiconductor material, such as the elemental silicon, forms its own Si phase that is dispersed in the matrix. This phase can contain multiple metalloids or metals (but metals only up to 50% by weight, better no more than 20% by weight; relative to the weight fraction of the additional component). In this context, the heating substrate material shows no open porosity, but no more than closed porosity of less than 0.5% and has a specific density of at least 2.19 g/cm³. It is therefore well-suited for applications in which purity or gas tightness of the heating substrate material are important.

The heat absorption of the heating substrate material depends on the fraction of the additional component. The weight fraction of silicon may therefore be at least 0.1%. On the other hand, the silicon volume fraction being high can have an adverse effect on the chemical and mechanical properties of the quartz glass matrix. Taking this into consideration, the weight fraction of the additional component consisting of silicon may be in the range of 0.1 to 5%.

Particularly high emissivity can be attained if the silicon is present as a separate silicon phase and includes a non-spherical morphology with mean maximum dimensions of less than 20 μm, but in certain embodiments of more than 3 μm.

In this context, the non-spherical morphology of the silicon phase also contributes to the heating substrate material having high mechanical strength and a low tendency to form cracks. The term “maximum dimension” shall refer to the longest extension of an isolated area of the silicon phase as visible in a microphotograph. The mean mentioned above is the median of all longest extensions in a microphotograph.

Components made of a composite material with a matrix made of quartz glass and having a silicon phase embedded in it are known. According to WO 2015067688 A1, these are used to fabricate, for example, reactors, fittings or wafer holders for use in an oxidation or annealing process, epitaxy or chemical gas phase deposition. For use as a thermally activatable furnace lining material according to exemplary embodiments of the present invention, the heating substrate has a printed conductor made of an electrical resistor material placed on it, which may be provided in the form of a burned-in thick film layer.

Such thick film layers are generated, for example, from resistor paste by means of screen printing or from metal-containing ink by means of inkjet printing, and are subsequently burned-in at high temperature. The printed conductor may include a precious-metal-containing material that does not lead to any service-life-reducing, oxidizing reaction products when exposed to air, such as, for example, of platinum, gold, silver, and alloys based thereon.

With regard to the temperature distribution being as homogeneous as possible, it has proven to be advantageous to provide the printed conductor as a line pattern, which covers the contact surface in such a way that an intervening space of at least 1 mm, and in some embodiments at least 2 mm, remains between neighbouring sections of printed conductor.

The printed conductor extends, for example, in a spiral-shaped or meandering line pattern. The absorption capacity of the heating substrate material being high enables homogeneous emission even if the printed conductor occupation density of the heating surface is comparably low. A low occupation density is characterized in that the minimum distance between neighbouring sections of printed conductor is 1 mm or more, and in some embodiments 2 mm or more. The distance between sections of printed conductor being large prevents flashover, which can occur, in particular, upon operation at high voltages in a vacuum.

A particularly advantageous embodiment of the infrared emitter according to the invention is characterized in that the heating substrate includes a surface facing the printed conductor and in that at least a part of the surface is configured with a cover layer made of porous quartz glass, whereby the printed conductor is embedded, at least in part, in the cover layer.

In this context, the cover layer made of opaque quartz glass serves as a diffuse reflector and concurrently protects and stabilizes the printed conductor. The effect and the production of a cover layer of this type made of opaque quartz glass are known and are described, for example, in WO 2006/021416 A1. It is generated from a dispersion that contains amorphous SiO₂ particles in a liquid. This is applied to the heating substrate surface facing the printed conductor, dried to form a green layer, and the green layer is sintered at a high temperature. The sintering of the green layer and the burning-in of the printed conductor may proceed in one and the same heating process.

An embodiment of the infrared heating unit according to the invention, in which the furnace lining includes a top side made of doped quartz glass, is suitable for many industrial heating applications.

Typically, the top side is a side of the process space that is directly opposite from the heating goods. In this context, the top side can concurrently serve as heated surface, in that it is provided as the heating substrate and is occupied by printed conductors for thermal excitation. In this context, the printed conductor may be situated on the side of the heating substrate facing away from the process space.

In the simplest case, the furnace lining consists fully of the doped quartz glass. The process space can be enclosed on all sides by the furnace lining made of IR black glass in this context. Having the process space being quasi-closed, the convection generated therein is also utilized for heating the heating goods. In another embodiment, the process space includes an open side, an opening or a passage, as is usually the case, for example, in continuous furnaces.

It has proven to be expedient to provide the heating substrate in the form of plates with a plate thickness of less than 5 mm.

Thin heating substrates have a low thermal mass and permit rapid temperature changes. Forced cooling is not required for this purpose.

A specific exemplary embodiment of the infrared heating unit according to the invention is designed in the form of a continuous furnace and, for the transport of heating goods through the process space, includes a transport facility that extends through the process space and includes support elements made of quartz glass on which the heating goods are supported.

Such an embodiment enables treatment of heating goods in a continuous pass-through. Due to the uniform IR emission and the high emissivity, the distance between the heating goods and the heating substrate can be small, which increases the irradiation intensity and the efficiency accordingly. The heating goods are exposed to a uniform (homogeneous) heating to (e.g., 1,000° C.), even while it is being passed through.

The support elements for the heating goods consist of quartz glass, in particular of undoped quartz glass or of a quartz glass doped with Al₂O₃. The support elements are characterized not only by their thermal stability and their chemical inertness, but also by high purity. This allows high purity of the overall system to be ensured. Moreover, rapid temperature change processes can be attained even without cooling such that it is possible to forego flowing coolant media, which are typically associated with problematic particle transport of possible impurities. The infrared heating unit according to the invention is therefore well-suited for use in semiconductor manufacturing processes.

In certain embodiments, the transport facility includes two continuous transport belts that run parallel to and at a distance from each other, whereby the support elements are provided in the form of quartz glass cylinders that bridge the distance between the transport belts and are connected to the transport belts in a torque-proof manner.

A traditional transport facility for continuous furnaces in the form of a roller conveyor also includes two continuous transport belts that extend parallel to and at a distance from each other. Circular cylinders are placed in the radial cross-section transverse to the conveying direction and are made to rotate by friction by the transport belts such that heating goods supported on the rollers are transported through the process space by the rolling motion without the rollers themselves moving along in the transport direction. In contrast, the transport facility according to exemplary aspects of the invention has the quartz glass cylinders connected, by their ends, to the transport belts such that they cannot rotate. Due to the support on the two transport belts such that no rotation is possible, vibrations are minimized; and, in addition, the formation of particles by abrasion is reduced. A particularly well-suited embodiment of the support elements are so-called commercial “twin tubes” with a twin tube geometry, in which two parallel tube-shaped compartments are separated by an intervening fin. Twin tubes made of quartz glass include high break resistance resulting from this material, which is increased even more by the special geometry of the twin tube.

Further improvement with respect to the infrared heating unit is attained if the quartz glass cylinders include a coating made of opaque quartz glass.

The coating made of opaque quartz glass acts as a diffuse reflector for infrared radiation and thus contributes to the efficiency of the heat treatment. The coating covers the quartz glass cylinders fully or partly; it can possess a certain roughness, which would counteract any sliding of the heating goods.

The surface lining of the infrared heating unit according to the invention may include at least one heated side, whereby the heating facility is designed for generating a total power density per surface area in the range of more than 100 kW/m² per heated side.

The furnace lining designed as a heating substrate currently serves as an optional heating surface. For example, in a furnace with rectangular bordering walls that is closed on all sides, the furnace lining has six planar sides. Considering six sides that are designed and used as heating surface, the overall power density per surface area n is at least 6×100 kW/m², and in some embodiments at least 6×200 kW/m². Considering a continuous furnace with a rectangular cross-section, which is designed to have optional furnace linings serving as heating surface on three sides, the overall power density per surface area, for example, is at least 3×100 kW/m², and in some embodiments at least 3×200 kW/m².

Referring now to the exemplary embodiment of the invention shown in the drawings, FIGS. 1-2 show an embodiment of the infrared heating unit according to the invention in the form of a continuous furnace 1 with a furnace housing 2, in which is situated a process space 3 for heat treatment, and which is fitted with a transport system 4 for continuous transport of heating goods 5 through the process space 3.

The transport system 4 includes two continuous transport belts 4.1 that are situated parallel to and at a distance from each other, and support elements in the form of twin tubes 4.2 made of transparent quartz glass, which bridge the distance between the transport belts and are held thereon such that they cannot rotate, on which the heating goods 5 are supported during the heat treatment.

A detail view of the torque-proof bracketing of the twin tubes 4.2 is shown in FIG. 3. A clamping spring 6 encompasses the end of a twin tube 4.2 and affixes it in a torque-proof position on one of the transport belts (not shown here). The clamping spring 6 is connected to elbows, which in turn are secured to one of the transport belts 4.1.

The exemplary dimensions of the twin cubes 4.2 in terms of length×width×height (L×W×H) are 1,000 mm×34 mm×14 mm; the wall thickness is approximately 2 mm. The distance of the twin tubes 4.2 from each other varies depending on the weight and geometry of the heating goods 5. For use of the device according to the continuous furnace 1 for burn-in of a gold layer on quartz glass tubes for lamp manufacturing, it is common to set the distance between the twin support tubes, for example, to 150 mm.

FIG. 2 shows a somewhat magnified cross-sectional view, as compared to FIG. 1, of the continuous furnace 1 1 according to the invention affording a view into the process space 3. The process space has, for example, a length of 2,000 mm, a width of 420 mm, and a height (calculated from the floor area 3.3 to the furnace lining in the ceiling area 3.1) of 145 mm.

The process space 3 is surrounded by a single-layered thermal insulation 7 in the ceiling area 3.1, side area 3.2, and floor area 3.3. The insulation 7 includes a refractory high temperature mat based on aluminium oxide and silicon oxide; it includes a thickness of 25 mm. Top side 3.1 and side wall 3.2 of the process space 3 are fully lined with plates made of a composite material (IR black glass), in which a crystalline phase made of elemental silicon is fine dispersed in a matrix made of quartz glass. In the sealing plate 9 bordering the top side, the rear side of a substrate 10 (see FIG. 4) made of the composite material is in contact with a meander-shaped printed conductor 11 that is embedded in a reflector layer 12 made of opaque quartz glass. The ceiling plate 9 concurrently serves as infrared emitter and is illustrated in more detail by means of FIG. 4.

The continuous furnace 1 according to exemplary embodiments of the invention is operated with a continuous electrical power of, for example, 20 kW and is used for a continuous sintering process. For this purpose, components coated with gold on the top side, for example, quartz tubes with dimensions of L×W×H=1,000×34×14 mm, are placed on the twin tubes 4.2 of the transport system 4 for burn-in of the coating, and are guided through the hot process space 3 at a rate of 200 mm/min. A total of 10 tubes are placed next to each other resulting in a throughput of approximately 100 tubes per hour. The exemplary continuous furnace 1 includes a height clearance of 100 mm (distance between the twin tubes 4.2 of the transport system and the ceiling area 3.1) and a width clearance of 420 mm. The free distance between the ceiling plate 9 and the heating goods depends on the height dimension thereof and is approximately 50 mm. A cooling device is not required and is not provided either.

After passage of the device 1, the coating on the tubes has a visually homogeneous surface with very good surface adhesion. The adhesion of the gold to the surface was determined using the adhesive tape tear-off test. The test encompasses applying a commercially available adhesive tape, for example, a Scotch adhesive tape made by 3M, onto the gold-coated surface and then tearing the tape off suddenly in one motion. If the adhesive strength of the gold is insufficient, metallic residues will be seen to remain on the adhesive surface of the tape. The metal-coated surface showed no impairment by particles or foreign substances whatsoever, since the continuous furnace 1 according to the invention utilizes no moving support elements that might release particles due to friction on the heating goods 5 or on other surfaces. Moreover, the process space 3 essentially includes surfaces made of quartz glass such that the process works free of contamination and does not generate particles in this area either.

FIG. 4 shows a schematic view of the ceiling plate 9 that closes off the process space in an upward direction. It is rectangular in shape and has lateral dimensions of 105 mm×105 mm and a plate thickness of 2.5 mm. The substrate 10 made of the composite material has a visually translucent effect to the eye. Upon microscopic inspection, it shows no open pores and at most closed pores with maximum mean dimensions of less than 10 μm. Non-spherical phase areas of elemental silicon (Si phase) are homogeneously distributed in the matrix. The weight fraction of the Si phase is 5%. The maximum mean dimensions of the Si phase areas (median) are in the range of approximately 1 to 10 μm. The composite material is gas-tight, it has a density of 2.19 g/cm³ and it is stable on air up to a temperature of approximately 1,200° C.

The embedded Si phase contributes not only to the overall opacity of the composite material, but also has an impact on the optical and thermal properties of the composite material. The composite material shows high absorption of heat radiation and high emissivity at high temperature.

At room temperature, the emissivity of the IR black glass is measured using an integrating sphere. This can be used to measure the spectral hemispherical reflectance R_(gh) and the spectral hemispherical transmittance T_(gh) from which the normal emissivity can be calculated. The emissivity at elevated temperatures are measured in the wavelength range from 2 to 18 μm by means of an FTIR spectrometer (Bruker IFS 66v Fourier Transformation Infrared (FTIR)) to which a BBC sample chamber is coupled by means of an additional optical system, applying the above-mentioned BBC measuring principle. In this context, the sample chamber is provided with thermostatted black body environments in the semi-spheres in front of and behind the sample holder, and with a beam exit opening with a detector. The sample is heated to a predetermined temperature in a separate furnace and, for the measurement, is transferred into the beam path of the sample chamber with the black body environments set to the predetermined temperature. The intensity detected by the detector is composed of emission, reflection, and transmission portions, namely intensity emitted by the sample itself, intensity that is incident on the sample from the front hemisphere and is reflected by the sample, and intensity that is incident on the sample from the back hemisphere and is transmitted by the sample. Three measurements are performed to determine the individual parameters (i.e., the degrees of emission, reflection, and transmission).

The degree of emission measured on the composite material in the wavelength range of 2 to approximately 4 μm is a function of the temperature. The higher the temperature, the higher is the emission. At 600° C., the normal degree of emission in the wavelength range of 2 to 4 μm is above 0.6. At 1000° C., the normal degree of emission in the entire wavelength range of 2 to 8 μm is above 0.75.

The printed conductor 11 is generated from a platinum resistor paste on the top 13 of the substrate 10. Both ends of the printed conductor have clamps for the supply of electrical energy welded to them. Printed conductor 3 shows a meandering profile that covers a heating surface of the ceiling plate 9 so tightly that an even distance of 2 mm remains between neighbouring sections of printed conductor. In the cross-section shown, the printed conductor 11 has a rectangular profile with a width of 1 mm and a thickness of 20 μm. It is in direct contact with the top 13 of the substrate 10 such that maximum heat transmission into substrate 10 is attained. The opposite bottom side 14 borders the process space 3 and concurrently serves as an emission surface for heat radiation. The direction of emission is indicated by direction arrow 15.

A reflector layer 12 made of opaque, pore-containing quartz glass is applied to the top 13 of the substrate 10 that faces away from the process space. It has a mean layer thickness of approximately 1.7 mm, is characterized by the absence of cracks and a high density of approximately 1.7 g/cm³, and is thermally stable at temperatures up to and above 1,100° C. The reflector layer 12 covers the printed conductor 11 completely and thus shields it from ambient chemical or mechanical influences.

In the embodiment of the continuous furnace 1 shown in FIGS. 1-2, only the ceiling plate 9 and the two side walls 3.2 of the process space 3, which are situated opposite from each other, consist of the IR black glass, whereas the floor below the support elements of the transport system 4 is formed by a parallel arrangement of twin tubes with no heating filament. The twin tube arrangement is provided with a reflector layer made of opaque quartz glass on its side facing the process space 3. The reflector layer includes small quartz glass particles with a diameter in the range of approximately 10 nanometres to 50 micrometers. The firmly sintered and correspondingly porous SiO₂ material, whose pores are filled with air, has an enormous surface area of approx. 5 m/g of the material due to the tiny structures. The surface being this large promotes rapid indirect heating of the air in the pores via the direct heating of the quartz glass by infrared radiation.

In another embodiment of the continuous furnace, both the linings of the ceiling and side walls and the floor consist of IR black glass and are provided on the rear side with printed conductors in the way described for the ceiling plate 9. Therefore, they can basically be operated as IR emitters (according to need).

In as far as the same reference numbers are used in FIGS. 2-4 as in FIG. 1, these denote components and parts that are structurally identical or equivalent as illustrated in more detail by means of the description of FIG. 1.

The infrared heating unit according to the invention enables a very uniform (homogeneous) heating by infrared radiation to approximately 1,000° C. in the continuous process. Due to the very homogeneous emission and high emissivity, the distance between the heating goods and the IR emitter element can be kept small, which strongly increases the irradiation intensity, and the energy efficiency increases accordingly. In this context, the efficiency of heating is at an unprecedented degree of efficiency, which is based on the quasi-black lining of the process space with an emissivity of more than 0.75 (at 1,000° C.). Due to the high emissivity, the absorbed backscatter from the process space is immediately returned to the process space for heating of the product such that losses are being minimized. Having the process space being quasi-closed, the convection generated therein is also utilized for heating. The high power density in excess of 200 kW/m² results mainly from the distance between the heating goods and the IR emitter being low.

Much like the process space, the conveying system being made of transparent quartz glass promotes the high purity of the overall system. In terms of its purity, the IR heating unit is well-suited for semiconductor applications. Moreover, there is no cooling required such that there is no particle motion effected by fans.

A method for producing the ceiling plate 9 shall be illustrated in more detail in the following by way of an example.

The production utilizes the slurry casting procedure described in WO 2015067688 A1. Amorphous quartz glass grains are purified in advance in a hot chlorination procedure making sure that the cristobalite content is below 1% by weight. Quartz glass grains with grain sizes in the range of 250 μm to 650 μm are wet milled with deionised water such that a homogeneous basic slurry with a solids content of 78% is formed.

Then the milling beads are removed from the basic slurry and a charge of silicon powder is admixed until a solids content of 83% by weight is attained. The silicon powder predominantly contains non-spherical powder particles with a narrow particle size distribution whose D₉₇ value is approximately 10 μm and whose fine fraction of particle sizes of less than 2 μm was removed in advance.

The slurry filled with the silicon powder is homogenised for another 12 hours. The silicon powder accounts for a weight fraction of the total solids content of 5%. The SiO₂ particles in the ready-homogenised slurry have a particle size distribution that is characterised by a D₅₀ value of approximately 8 μm and a D₉₀ value of approximately 40 μm.

The slurry is cast in a die of a commercial die-casting machine and dewatered using a porous plastic membrane to form a porous green body. The green body has the shape of a rectangular plate. To remove bound water, the green body is dried at approximately 90° C. for five days in an aerated furnace. After cooling, the porous blank thus obtained is processed mechanically to be close to the final dimension of the quartz glass plate to be produced, which has a plate thickness of 2.5 mm. For sintering, the blank is heated over the course of one hour to a heating temperature of 1390° C. in a sintering furnace in the presence of air and maintained at this temperature for 5 h.

The quartz glass plate (10) thus obtained consists of a gas-tight composite material with a density of 2.1958 g/cm³, in which non-spherical regions of elemental Si phase that are separated from each other and whose size and morphology correspond essentially to those of the Si powder used in the process are homogeneously distributed in a matrix made of opaque quartz glass. The maximum mean dimensions (median) are in the range of approximately 1 to 10 μm. The matrix looks translucent to the eye. Upon microscopic inspection, it shows no open pores and at most closed pores with maximum mean dimensions of less than 10 μm; the porosity calculated based on the density is 0.37%. The composite material is stable on air up to a temperature of approximately 1200° C.

The quartz glass plate (10) is being polished on the surface such that a mean surface roughness Ra of approximately 1 μm is established. The meander-shaped printed conductor 11 is applied to its polished top 5 by means of a screen-printing procedure. A commercial platinum-containing resistor paste is used for this purpose.

After the printed conductor 11 is dried, a layer of slurry is applied to the top 13 of the quartz glass plate (10). This slurry is obtained by modification of the basic SiO₂ slurry of the type described above (without added silicon powder) by admixing amorphous SiO₂ grains in the form of spherical particles with a grain size of approximately 5 μm to the homogeneous stable basic slurry until a solids content of 84% by weight is attained. This mixture is homogenized for 12 hours in a tumbling mill rotating at a rate of 25 rpm. The slurry thus obtained has a solids content of 84% and a density of approximately 2.0 g/cm³. The SiO₂ particles in the slurry obtained after milling of the quartz glass grains have a particle size distribution that is characterized by a D₅₀ value of approximately 8 μm and a D₉₀ value of approximately 40 μm.

Cleaned in advance in alcohol, the quartz glass plate 10 is immersed in said slurry for a few seconds. As a result, a homogeneous slurry layer with a thickness of approximately 2 mm is formed on the quartz glass plate 10. After wiping the bottom side 146, the slurry layer is dried initially for approximately 5 hours at room temperature and subsequently on air by means of an IR emitter. The dried slurry layer is free of cracks and has a mean thickness of slightly less than 2 mm.

Subsequently, the dried printed conductor and the dried slurry layer are burned-in and/or sintered on air in a sintering furnace. The heating profile includes a heating temperature of 1200° C. The holding time is two hours in the exemplary embodiment. Subsequently, the printed conductor 11 is burned-in and the slicker layer 12 is opaque, but visually dense and largely free of bubbles.

The infrared heating unit according to the invention includes heating elements made of IR black glass has a low thermal mass and therefore a rapid reaction time (<10 s) and—depending on the occupation density of the printed conductor—is very homogeneous in its emission. The IR black glass affords not only very good emission characteristics (broadband, high emission at high temperatures), but it also possesses the purity that is required in semiconductor processes. Moreover, the tiles require no further cooling.

The infrared heating unit fitted with a planar, plate-shaped IR emitter of the type described herein can be used in the printing industry in order to implement high process speeds (>100 m/s) at very high power density (>100 kW/m² per heated side) and low distances to the heating goods (<5 mm) that are passed by the IR emitter, for example, on a belt. An electrically conductive metal layer can be applied to it by means of screen printing procedures.

3D printing is another area of application. Due to the high power per unit area and the high working temperature (up to approximately 1,000° C.), printed metallic powders can be partially sintered and/or sintered.

In this context, the infrared heating unit according to the invention can be designed, for example, as a chamber furnace that is closed on all sides. A chamber furnace with a rectangular cross-section possesses six planar furnace linings that are provided as heating substrates and can generate a power density of at least 600 kW/m² (at least 100 kW/m² per heated side) and even clearly more (at least 200 kW/m² per heated side).

Basically, the excellent properties of quartz glass for the applications on hand are coming to bear, for example, the high temperature cycling resistance, the chemical inertness, the high strength, and the purity.

Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention. 

1. An infrared heating unit with a furnace comprising: a housing that accommodates a process space, and a heating facility, whereby the process space is bordered, at least in part, by a furnace lining made of quartz glass, wherein the heating facility is formed by at least one heating substrate defining a contact surface that is in contact with a printed conductor that is made of a resistor material that is electrically conductive and generates heat when current flows through it, whereby the heating substrate includes doped quartz glass into which an additional component that absorbs in the infrared spectral range is embedded, the heating substrate forming at least a part of the furnace lining.
 2. The infrared heating unit according to claim 1, wherein the furnace lining is fully made of the doped quartz glass.
 3. The infrared heating unit according to claim 2, wherein a distance between the heating goods and the heating substrate is set to less than 5 mm.
 4. The infrared heating unit according to claim 2, the additional component is elemental silicon and is embedded in an amount that effects, in the heating substrate material for wavelengths between 2 and 8 μm, an emissivity ε of at least 0.6 at a temperature of 600° C. and an emissivity ε of at least 0.75 at a temperature of 1,000° C.
 5. The infrared heating unit according to claim 4, wherein an amount of the additional component, relative to the weight of the heating substrate, is in the range of 0.1 to 5% by weight.
 6. The infrared heating unit according to claim 1, wherein the printed conductor is provided as a burned-in thick film layer and as a line pattern that covers the contact surface appropriately such that an intervening space of at least 1 mm remains between neighbouring sections of the printed conductor.
 7. The infrared heating unit according to claim 1, wherein the contact surface is occupied, at least in part, by a cover layer made of porous quartz glass.
 8. The infrared heating unit according to claim 1, wherein the heating substrate is provided to be plate-shaped and to have a plate thickness of less than 5 mm.
 9. The infrared heating unit according to claim 1, wherein a transport facility for transport of heating goods through the process space is provided that extends through the process space and includes support elements made of quartz glass on which the heating goods are supported.
 10. The infrared heating unit according to claim 9, wherein the transport facility includes two continuous transport belts that run parallel to and at a distance from each other, whereby the support elements are provided in the form of quartz glass cylinders that bridge the distance between the transport belts and are connected to the transport belts in a torque-proof manner.
 11. The infrared heating unit according to claim 10, wherein the quartz glass cylinders include a coating made of opaque quartz glass.
 12. The infrared heating unit according to claim 1, wherein the furnace lining includes at least one heated side and in that the heating facility is designed for generation of an overall power density per surface area in the range of more than 100 kW/m² per heated side.
 13. The infrared heating unit according claim 1, wherein the printed conductor is provided as a burned-in thick film layer and as a line pattern that covers the contact surface appropriately such that an intervening space of at least 2 mm remains between neighbouring sections of printed conductor.
 14. The infrared heating unit according to claim 2, wherein the contact surface is occupied, at least in part, by a cover layer made of porous quartz glass.
 15. The infrared heating unit according to claim 1, wherein the contact surface is occupied, at least in part, by a cover layer made of porous quartz glass.
 16. The infrared heating unit according to claim 1, wherein the distance between the heating goods and the heating substrate is set to less than 5 mm.
 17. The infrared heating unit according to claim 1, wherein the additional component is elemental silicon and is embedded in an amount that effects, in the heating substrate material for wavelengths between 2 and 8 μm, an emissivity ε of at least 0.6 at a temperature of 600° C. and an emissivity ε of at least 0.75 at a temperature of 1,000° C.
 18. The infrared heating unit according to claim 17, wherein an amount of the additional component, relative to the weight of the heating substrate, is in the range of 0.1 to 5% by weight.
 19. The infrared heating unit according to claim 1, wherein the furnace lining is fully made of the doped quartz glass, wherein the distance between the heating goods and the heating substrate is set to less than 5 mm, wherein the additional component is elemental silicon and is embedded in an amount that effects, in the heating substrate material for wavelengths between 2 and 8 μm, an emissivity ε of at least 0.6 at a temperature of 600° C. and an emissivity ε of at least 0.75 at a temperature of 1,000° C.
 20. The infrared heating unit according to claim 19, wherein an amount of the additional component, relative to the weight of the heating substrate, is in the range of 0.1 to 5% by weight. 